Dental modeling system and method

ABSTRACT

A method for simulating tooth movement utilizes electronic modeling to represent the teeth. Instant centers of rotation are determined and projected paths of movement are plotted. The electronic model provides improved precision and provides a method for simulating movement in three dimensions. The movement from growth and/or correction is shown with the electronic model. The simulation provides for improved correction.

This application is being filed on 4 May 2007, as a PCT InternationalPatent application in the name of Bruce Willard Hultgren, a citizen ofthe U.S., applicant for the designation of all countries, and claimspriority to U.S. Provisional Patent Application No. 60/797,911, filedMay 4, 2006.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method and system for simulatingmovement of a patient's jaw and teeth.

2. Description of the Prior Art

In the field of orthodontics, simulation is important for treatment andcorrection. If the manner in which the teeth and jaw may form and growis known, the dental practitioner is better able to diagnose problemsand identify potential problems. In addition, the limits of moving teeththrough braces and other correction techniques become more apparent andtreatment may be carried out with more precision and greater chance ofsuccess.

Prior simulation techniques consisted of the dental practitionerobserving the patient's teeth and utilizing the practitioner's knowledgeand judgment to predict how the teeth would move by estimating theeffects of braces and other corrective techniques. Although usuallysuccessful in correcting at least some of the structural problems,greater success rates are possible with additional information andknowledge about the patient's likely tooth movement both with andwithout correction.

Efforts to model tooth movement have been developed that proved to be agreat aid for practitioners. Some models involve linear tooth growth,especially for those in the lower jaw. However, studies and modelingreveal that tooth movement and growth is not usually truly linearlytranslational. Although such modeling is helpful, with greaterprecision, more accurate simulations could be achieved.

Other theories that have been developed involve identifying an imaginarycenter of rotation for a particular tooth. Such theories as stated inpapers by Pilgrim, Moses, Erdman and Hultgren, utilized instant centersof rotation to predict tooth movement along an arc centered on theinstant center of rotation. It is believed that such a model istypically a better predictor of actual tooth movement than a model usingstraight line movement.

A further model simulates tooth movement as being governed by a linkage.The model predicts growth that is not straight line, but the arc alongwhich the tooth moves is not centered on a single center of rotation andtherefore the tooth behaves as if it was a segment of a linkage, forexample, following a path as if it were mounted on one link of a fourbar linkage.

Although such models have proven to be successful in providing guidancefor predicting tooth growth and movement, implementation in a practicalmanner has proven difficult. Current techniques do not provide foraccurately tracking tooth movement with sufficient precision and theirrelationship to the tooth's reference points. Markers and/or landmarksare necessary to ensure that the movement of the tooth or of a specificportion of the tooth are tracked. While such markers provide forimproved modeling and tracking, the insertion and placement of suchmarkers and tracking of the markers relative to one another is intrusiveand only allows tracking of the marker rather than the entire tooth. Aportion of the tooth moves with the marker, but it is possible that theorientation in one or more planes may have changed or other portions ofthe tooth may have moved in a different manner.

A further problem with such markers and/or landmarks is the precisionpossible in tracking the changes in position. Heretofore, tracking ofmarkers in teeth has generally been accomplished by taking x-rays of themarkers and teeth and outlining the teeth and/or markers to establish acurrent position. The insertion and removal of markers is intrusive andinconvenient for the patient, requiring an additional procedure. Whilesuch methods do provide for tracking of position and changes, theaccuracy of such methods is decreased due to quality and resolutionavailable for the images and the inaccuracy from manually outlining andprecisely locating the markers and/or structural landmarks. As theaccuracy and precision are critical for predicting small movements andin locating instant centers of rotation, such tools may not be anacceptable simulator of the teeth and their movement. Due to the precisemeasurements and the small distances involved, even minor errors canhave a drastic adverse affect on the position of an instant center ofrotation and therefore, the entire model and movement simulation,increasing the possibility for improper diagnosis and treatment.

Techniques have also been developed in orthodontics modeling to create adigital three dimensional model by scanning a casting of a patient'steeth and gums.

The digital EMODEL™ digital model system provides a precise highresolution digital map of the teeth and gums, but its use has beenlimited in creating prostheses. Such three dimensional digital imageshave been used as a starting point in treatment, but have not been usedto project and simulate growth and movement.

It can be seen then that a new and improved system is needed thatovercomes the problems and deficiencies associated with the prior artmodeling systems and methods. A system and method is needed thatprecisely models position and accurately simulates movement of thepatient's jaw and teeth. In addition, such a system and method shouldprovide minimal intrusion for the patient. Such a system and methodshould also be practical to utilize and should achieve sufficientprecision and reliability for accurate simulation by the dentalpractitioner to diagnose problems and prescribe proper treatment. Thepresent invention addresses these as well as other problems associatedwith simulating tooth growth and movement.

SUMMARY OF THE INVENTION

The present invention is directed to a system and method for modelingtooth movement. Electronic modeling is used to create a precise andaccurate three dimensional virtual model of the teeth.

In creating an electronic virtual model, a casting of the patient'steeth is taken and a plaster model created. A scanner is used todigitize a three dimensional image of the patient's mouth for modeling.Alternatively, three dimensional high resolution digital scanners, suchas CT scanners, intra-oral scanners, cone beam scanners or other digitalcapture devices may scan the patient's facial structure to create athree dimensional virtual model. A computer database stores the digitalimage and the image may be viewed as a three dimensional virtual modelthat may be manipulated on the computer screen. Moreover, the systemprovides for separating the teeth into discrete elements for modelingmovement of an individual tooth and for modeling movement of all teethto help in providing proper diagnosis for treatment and correction. Theimages may be manipulated to model projected growth or movement overtime.

According to the present invention, modeling of tooth movement isaccomplished by identifying an instant center of rotation for aparticular tooth or other structure of the patient's jaw. The movementthat the tooth would follow at that point in time is an arc centeredabout the instant center of rotation. The instant center of rotation canbe plotted in three dimensions so that the model tooth movement followsthe surface of an imaginary sphere centered about the instant center ofrotation. As the image is digitized, the teeth or other structures andtheir respective instant centers of rotation can be mathematicallyrepresented with a set of coordinates so that the plotting of movementis accomplished with great precision. Locating the instant center ofrotation is accomplished utilizing a number of factors based on thepatient's tooth and jaw physiology and the knowledge and experience ofthe dental practitioner. It can be appreciated that the instant centerof rotation varies from patient to patient and an instant center may beabove or below the position of the tooth as well as to one side or theother, or forward or behind a tooth. The position may be quite close tothe tooth or may approach infinity so that the virtual toothsubstantially models straight line movement.

The position of the instant center of rotation may be moved to reflectdifferent types and sets forces applied to the tooth, such as movementto changing position of surrounding teeth or the application of bracesto the tooth. Instant centers may also be determined for largerstructures in the mouth or jaw whose movement may be simulated.Moreover, the forces of growth and passage of time are typicallydifferent than the forces acting on the tooth or structure due tocorrection. Therefore, depending on the evaluation and analysisrequired, different instant centers of rotation may be determined foranalyzing different sets of kinematic and dynamic factors. As an instantcenter of rotation shifts, the projected path also shifts and the radiusfrom the instant center to the projected arc may lengthen or shorten.With the digital image stored, the treatment and the modeling providesfor application of varied forces and in various directions withdifferent projected paths of movement so that the diagnosis andtreatment may be optimized.

The present invention also contemplates modeling for movement of someteeth wherein a tooth substantially pivots about two axes of rotationand models the movement of a virtual four bar linkage. The modeling issimilar to the modeling for rotation about a single axis, but two pointson the tooth or other structure are plotted. Using the same techniquesdiscussed above, the instant centers of rotation are located for bothpoints on the tooth. The entire tooth is then moved with each plottedpoint on the tooth following a path of movement along an arc centered onthe associated instant center of rotation, so that the toothsubstantially follows the movement of a four bar linkage type model.

These features of novelty and various other advantages that characterizethe invention are pointed out with particularity in the claims annexedhereto and forming a part hereof. However, for a better understanding ofthe invention, its advantages, and the objects obtained by its use,reference should be made to the drawings that form a further parthereof, and to the accompanying descriptive matter, in which there isillustrated and described a preferred embodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings, wherein like reference numerals andletters indicate corresponding structure throughout the several views:

FIG. 1 is a perspective view of a three-dimensional digital model of apatient's dentition;

FIG. 2 is a diagrammatic view of a system for modeling tooth movementaccording to the principles of the present invention;

FIG. 3 is a diagrammatic view of a portion of the virtual model shown inFIG. 1 with an instant center of rotation shown for a selected tooth;

FIG. 4 is a diagrammatic view of a projected path of movement for aselected portion of the selected tooth shown in FIG. 3;

FIG. 5 is a diagrammatic view of a projected path of movement forgrowth;

FIG. 6 is a perspective view of a projected path of movement forcorrection (with the instant center shifted-show 2);

FIG. 7 shows a flow chart of the steps to create a digital model;

FIG. 8 is a flow chart showing the steps to model tooth movementaccording to the principles of the present invention;

FIG. 9 is a diagrammatic view of the virtual model shown in FIG. 3 attwo points in time to determine an instant center of rotation; and

FIG. 10 is a diagrammatic view of a tooth with two points plotting apath of movement about an instant center of rotation for each point.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings, and in particular to FIG. 1, there isshown a digital image of a patients dentition 20 including an upper archhaving a portion of the upper gums 22, and the exposed surfaces of theupper teeth 26. In a similar manner, a lower arch including a portion ofthe teeth and lower gums 24 is also created in a digitized model alongwith the exposed upper surfaces of the lower teeth 28. Such an image ofthe dentition 20 is digitized and provides a virtual three dimensionalimage of the patient's teeth for diagnosis, treatment and correction.Such a method and system for creating such a virtual image of thepatient's mouth is shown and described in U.S. Pat. No. 6,579,059,incorporated herein by reference. One method of creating digital imagesis taking a casting of the patients teeth and creating a plaster modeland scanning the plaster cast. The digitizing system 70 is shown in FIG.2 and includes a scanner 72 that passes a laser over the plaster modelheld by fixture 74. A rotatable support 76 mounts on a base 78 andprovides for moving the plaster model relative to the scanner so thatthe outer contour of the patient's teeth and gums can be stored in adigital format of a memory device 86. A central processor 82 providesfor converting the data from the scanner to a digital image of theexterior of the patient's teeth and gums. An input device 80 providesfor manipulating images 88 and a display 84 provides a screen forviewing the virtual image of the teeth and gums. It can be appreciatedthat the digital image provides for modeling with each tooth representedby sets of coordinates in three dimensions so that the tooth may bemanipulated independently such as to model tooth movement. Moreover,when a digitized virtual model is created in three dimensions,mathematical calculations may be performed and then displayed to providethe dentist or orthodontist with sufficient precision for meaningfuldiagnosis and treatment. Mapping in three dimensions and greaterprecision provide for meaningful treatment that is not possible withprior methods based on markers and/or x-rays.

More recently, high resolution three dimensional scanners have beendeveloped that provide direct three dimensional craniofacial imagingthat may be used to create a digital representation of a patient's teethwith a high degree of precision without plaster casts and without usinglandmarks or markers. Such scanners include cone beam tomographyscanners, intra-oral scanners, CT scanners and other technologies thatprovide high resolution precision three dimensional digital models.Examples of commercial available scanners include cone beam scannerssold under the trade names ILUMA™ from Kodak and i-CAT from ImagingSciences International. Cone beam tomography scanners are well suitedfor such applications as they emit lower radiation than traditionaldental x-rays and may eliminate the need for creating plaster casts.

Referring now to FIG. 3, there is shown a portion of the patient'sdentition including representative upper incisors and molars 26 andlower incisors and molars 28. A selected tooth 30 having a complementarylower tooth 34 shown as an upper molar in FIG. 3 is modeled and movementof the tooth may be determined. It has been found that the path ofmovement of a tooth may be more accurately modeled as rotating about animaginary point, forming an axis of rotation, represented as centerpoint C1 in FIG. 4. Each tooth has various forces acting on the toothand other structure in the jaw that affect movement. A kinematic anddynamic analysis and synthesis is performed based on the patient'sfacial structures to determine the imaginary instant center. Animaginary instant center of rotation may be calculated for each tooth.In addition to an individual tooth, the jaw, groups of teeth and otherstructures in the jaw and mouth may also have an imaginary instantcenter point about which such structures move. Moreover, the forces dueto growth may act differently on the tooth or structure so that a toothor structure in a patient that is growing may have an instant center forcorrective or restorative procedures and a different instant center tosimulate movement for passage of time and growth. The center point C1may be represented as a set of X, Y and Z coordinates in the digitalmodel. The movement of the tooth 30 is limited by the gums, jaw,surrounding teeth and other structures and forces acting on it.Therefore, the movement of point 32 will occur along an arcing path P1centered on the instant center of rotation C1. The tooth will move alongthis path and correction, growth or other forces acting on the tooth maydirect it a distance along this path. The path may be charted along aset of coordinates in the digital model with great precision. Such avirtual model may be utilized to predict repositioning of the tooth orstructure from the passage of time and growth. In addition, a virtualmodel that may have a different instant center of rotation may beutilized to predict repositioning of the tooth or structure due toforces applied by the other teeth and structures in the mouth or theaddition of orthodontic structures. Predicting the distance along theprojected path P1 that a tooth will move, a new position of the tooth 30may be projected and also represented mathematically as a new set ofcoordinates stored in the memory device 86. Such a three dimensionalrepresentation may be shown and manipulated by the dental practitionerin selecting and optimizing treatment.

To determine a path of rotation for modeling the tooth and toothmovement, a point 32 on or in the tooth is selected as the tracing pointfor projecting a path of movement. An instant center of rotation isestablished with published methods and using the expertise andestimation from the dental practitioner based on features of the virtualimage of the dentition 20 as well as the type and overall structure ofthe patient's skull and jaw. The instant center is established byanalyzing the various static and dynamic forces acting on the tooth. Inone embodiment, the center point C1 is selected based on the occlusalplane of the teeth 30 and 32, the structure of the patient's jaw, andpatterns of movement generally associated with similar physiology.Centers of rotation have been previously charted from studies conductedusing prior art markers that provide guidance for the dentalpractitioner locating the instant center. Similar jaw structures forcomparison provide guidance to locate centers of rotation, so that theaxis may be accurately approximated with the available information andan accurate virtual model of the patient's dentition 20.

As shown in FIG. 7, the point of projection 32 is entered and stored asa set of coordinates in the memory device 86, shown in FIG. 2. Inaddition, the orientation of the remainder of the exposed portion of thetooth 30 is also entered and stored as a set of coordinates in threedimensions. This orientation is in relation to a radius R1 extendingfrom the point of projection 32 to the center C1. As the tooth movesalong the projected path of movement P1, the radius distance remainsconstant and the position and orientation of the tooth structure 30translates with respect to the radius R1. A direction of movement isalso determined by the dental practitioner using knowledge of thepatient's physiology and the forces applied by the selected treatment.

Based on the repositioning of the tooth 30 as it rotates about thecenter point C1, various treatments and/or decisions may be maderegarding alignment and positioning of the tooth. Moreover, byperforming the same analysis on other teeth, an overall treatment planmay be devised in a coordinated manner so that treatment is moreeffective.

Referring now to FIG. 5, in some instances, the rotational center of theplotted point 32 may move closer to the tooth due to changes in forcesapplied, such as for example, braces. For example, C1 may shift fromposition C1 to position C2. Therefore, the path of rotation moves fromthe projected path P1 to the projected path P2. The movement of theprojected path allows for recalculating movement and more precisediagnosis and treatment for more properly positioning the teeth.Although the center is shown to be moved closer to the tooth 30 in FIG.5, it can be appreciated that the center of rotation may shift in anydirection. Moreover, the instance center may be above, below, behind orin front of the tooth and that the illustrated center C1 and C2 and thecorresponding paths P1 and P2 and radial cords R1 and R2 are shown asexamples only. Positioning will depend upon the particular physiology ofthe patient's teeth and jaws.

Referring now to FIG. 6, there is shown a further example of a projectedpath and changes that may occur to move the instant center of rotation.For example, with some treatments, such as the application of correctivebraces to the tooth, the instant center of rotation may move from arotational axis shown at C1 in FIG. 4 to a rotational axis centered atC3, as shown in FIG. 6. The forces change the instant center of rotationand move the projected path of movement from path P1 to path P3.Although the rotational axis at the instant center C1 is moved fromslightly above and behind the tooth 30 to a location in front of andbelow the tooth 30 to rotational axis point C3, the instant center ofrotation may begin and end in any combination of places and it can bereadily appreciated that the locations are shown only as examples. Othersurrounding physiology and corrective devices may alter the center ofrotation and/or the projected path. In a method of the presentinvention, the change is factored as a force vector acting on the tooth.For patients where tooth growth will occur, a growth coefficient may befactored to determine how much movement will occur over a predeterminedperiod.

For some patients, the center of rotation may be positioned far from thetooth 30 and the length of the associated radius may approach infinityso that substantially linear motion is projected for the path ofmovement. The projected path of movement may be modeled with thedigitized virtual model to occur on all three planes so that theprojected center of rotation may also be positioned laterally from thepatient's face. Such projected path of movement is easier to plot andrepresent in a digitized virtual model with greater precision, such asshown in FIG. 1, and provides greater feedback and improved informationfor the dental practitioner to evaluate for proper treatment. Moreover,as the present invention provides for plotting paths of movement aboutvarious potential shifted centers of rotation, multiple differenttreatment approaches having different forces may be modeled. Therefore,different types of braces or other corrective measures may be evaluatedby plotting the different centers of rotation and the correspondingprojected path of movement. Modeling movement with precision leads tooptimizing tooth position, diagnosis and correction.

Referring now to FIG. 8, to create a projection of tooth movement aboutan instant center of rotation, the model of the casting of the tooth ismade and scanned into the memory 86 of the scanning system 80 as shownin FIG. 7. The patient's physiology and the virtual model are analyzedto locate an instant center of rotation for a particular tooth. Basedupon the instant center of rotation, a projected path of movement isplotted. Using a time and force factor, a direction and distance alongthe projected path of movement is selected. The tooth is thenrepositioned with the projection point located along the path ofmovement the selected distance in the selected direction. A digitalimage is then created showing the tooth at the new location. It can beappreciated that such an image can be superimposed over the old locationand combined with individual digital images of the other teeth andfacial structures to create a complete new three dimensional image ofthe patient's teeth to simulate movement. Many virtual images can besuperimposed to compare and optimize treatments.

Referring now to FIG. 9, the present invention also provides fortracking of tooth movement through time by comparing and mapping theposition of a tooth 30 over time. Such changes in position allow forfurther projection by locating a center of rotation by triangulating thecenter point from the tooth orientation at each of the positions. Suchtriangulation provides a center of rotation that can be used to plot apath of movement P4 about a center of rotation C4 and having a radialcord R4. The historical movement also provides for modifying theselected center of rotation and for reevaluating treatment.

Referring now to FIG. 10, the present invention also provides formodeling growth for movement that does not follow rotation about asingle center of rotation for a particular structure, such as a tooth100. The tooth 100 and its movement may be represented as having a firstpoint 102 following a path of movement centered about a first rotationalaxis C5 while a second point 104 of the tooth structure 100 may followmovement centered about a second instant center of rotation C6. Each ofthe locations 102 and 104 has a corresponding path of rotation, P5 andP6 respectively, and radial cords R5 and R6. In this manner, it can beappreciated that the movement of the tooth 100 over time may still beplotted with the tooth 100 having the same motion as a four bar linkage.It can be appreciated that the centers of rotation C5 and C6 act as theground or first link while the radial chords R5 and R6 represent twoother links. A tooth structure itself 100 represents a virtual fourthlink. It can be appreciated that the rotational center points C5 and C6may shift as explained above. The paths of movement P5 and P6 would alsoshift and movement of the two points 102 and 104 would follow adifferent four bar linkage, with the tooth reorienting and/or moving.Modeling techniques and treatment discussed above for a single instantcenter may also be employed for a tooth having movement behaving as avirtual four bar linkage.

It is to be understood, however, that even though numerouscharacteristics and advantages of the present invention have been setforth in the foregoing description, together with details of thestructure and function of the invention, the disclosure is illustrativeonly, and changes may be made in detail, especially in matters of shape,size and arrangement of parts within the principles of the invention tothe full extent indicated by the broad general meaning of the terms inwhich the appended claims are expressed.

1. A method of modeling tooth movement of a patient, comprising:creating a first virtual model of the patient's teeth; identifying afirst reference point on a first virtual tooth of the model; identifyinga second reference point on the patient's mandible; creating a secondvirtual model of the patient's teeth after a time period; tracking themovement of the first reference point from a first location to a secondlocation over the time period; tracking the movement of the secondreference point from a third location to a fourth location over the timeperiod; extending a first segment between the first location and thesecond location; extending a second segment between the third locationand the fourth location; projecting a first perpendicular bisector fromthe first segment; projecting a second perpendicular bisector from thesecond segment; determining an intersection of the first and secondbisectors, defining a rotational axis; plotting an arc centered on therotational axis along which the first reference point will travel.
 2. Amethod according to claim 1, wherein the computer projects toothmovement in the future and creates a third virtual model representing afuture tooth position.
 3. (canceled)
 4. A method of modeling a patient'stooth movement on linkage configurations, comprising: defining a groundpoint axis of rotation; adopting a first point on the patient's ramus asa first link pivot; adopting a second point on one of the patient'slower teeth as a second link pivot; tracing a first path of the firstpoint pivoting about the axis of rotation, tracing a second path ofrotation about the axis of rotation pivoting about the axis of rotation;moving the first point along the first path a first distance; applying arelative movement factor to the first distance to determine a seconddistance between the first point and the second point; moving the secondpoint along the second path until the second point is the seconddistance from the first point to arrive at a predicted location.
 5. Amethod according to claim 4, wherein the linkage is oriented withvirtual modeling.
 6. A method according to claim 4, wherein the groundpoint axis of rotation is determined with virtual models.
 7. A methodaccording to claim 4, growth factor to determine distance between twopivots.
 8. A method according to claim 4, wherein coefficient ofrelative growth is applied to the first distance to determine the seconddistance.
 9. A method according to claim 4, further adopting a segmentconnecting the axis of rotation and the first point as the first link;adopting and a segment connecting the axis of rotation and the secondpoint as the second link; rotating the first point along the first patha first angle; applying a relative movement factor to the first angle todetermine a second angle between the first segment and the secondsegment; moving the second link along the second path the second angleto arrive at a predicted location.
 10. A method according to claim 9,wherein the first link is a driven link.
 11. A method according to claim10, wherein the first link is on the patient's ramus.
 12. A method ofmodeling tooth movement, comprising: creating a first virtual model of atooth; creating a second virtual model of the tooth after a period oftime; identifying a rotational axis of the tooth; projecting an arcalong which the tooth will travel centered on the rotational axis.
 13. Amethod according to claim 12, wherein the virtual models are stored on acomputer, and wherein the computer performs identifying the rotationalaxis and projecting the arc.
 14. A method of according to claim 12,further comprising: making a first positioning location on a patient'smandible; tracking the movement of the positioning location over theperiod of time wherein the rotational axis of the tooth is based onpositions of the tooth and the positioning location before and after theperiod of time; projecting an arc along which the tooth will travelcentered on the rotational axis.
 15. A method according to claim 13,wherein the virtual models are stored on a computer, and wherein thecomputer performs the steps of identifying the rotational axis andprojecting the arc. 16.-17. (canceled)
 18. A method of modeling toothmovement, comprising: creating a virtual model of a patient's mouth;selecting an element for projecting movement; determining a firstinstant center of rotation associated with a first location on theelement; determining a second instant center of rotation associated witha second location on the element; tracing a path of movement through thefirst location and centered on the first instant center of rotation;tracing a path of movement through the second location and centered onthe second instant center of rotation; and tracing a path of movementfor the element with the first location centered on the first instantcenter of rotation and the second location centered on the secondinstant center of rotation.
 19. A method according to claim 18, whereinthe element defines a link in a virtual four-bar linkage.
 20. A methodaccording to claim
 19. wherein the pivots in the four-bar linkage are atthe first location, the second location, the first instant center ofrotation and the second instant center of rotation.
 21. (canceled)
 22. Amethod according to claim 18, wherein the virtual model comprises athree dimensional model.